Stigmatically imaging double focusing mass spectrometer

ABSTRACT

To provide for better energy definition in a stigmatically focussing mass spectrometer with directional and energy focussing, the ions to be analysed travel consecutively through an entry diaphragm, a first analyser, a diaphragm to limit the energy range (&#39;&#39;&#39;&#39;energy diaphragm&#39;&#39;&#39;&#39;), a second analyser, (the energy dispersion factor of which is equal but opposite to that of the first analyser) and an exit diaphragm. One analyser is a momentum analyser operating with a wedge-shaped magnetic field, and the other analyser is an energy analyser operating with an electric field. The momentum analyser comprises a magnet having mutually opposite plane pole-piece surfaces which are mutually inclined and have straight edges which are located at the interval Ao parallel to the intersection line of the planes containing the pole-piece surfaces. A device is provided for adjusting the magnetic field intensity BA at the interval A 2.10Ao from the intersection line to the value BA 1.35 p/Ae (e charge of the ions; p momentum of the ions to be detected). The diaphragms between which the impulse analyser is placed are arranged on the intersection line with a mutual interval of approximately 1.28 A, and the energy analyser, which forms a stigmatic image of the entry side one of the two diaphragms between which it is arranged upon the exit side one of the two said diaphragms, contains a spherical condenser or a cylindrical condenser through which the ions travel in planes which pass through the cylinder axis.

[ Sept. 25, 1973 STIGMATICALLY IMAGING DOUBLE FOCUSING MASS SPECTROMETER [75] Inventor: Helmut .l. Liebl, Eching, Germany [73] Assignee: Max-Planck-Gesellschaft zur Forderung der Wissenschaften e.V., Gottingen, Germany [22] Filed: June 21, 1971 {21] Appl. No.: 155,061

[30] Foreign Application Priority Data June 26. 1970 Germany P 20 31 811.0

[52] US. Cl 250/4l.9 ME, 250/49.5 P [51] Int. Cl. HOlj 39/34, B0ld 59/44 [58] Field of Search 250/419 ME, 49.5 P

[56] References Cited UNITED STATES PATENTS 3.445.650 5/1969 Liebl 250/419 OTHER PUBLICATIONS -Ion Microprobe Mass Analyzer By H. Liebl from the Journal of Applied Physics, Vol. 38, No. 13, Dec, 1967. pages 5277-5283. Simple Broad-Range Magnetic Spectrometer By J. S. OConnell from The Review of Scientific Instruments, Vol. 32, No. 12,Dec.. 1961, pages 1314-1316.

Primary Examiner-William F. Lindquist Attorney-Robert D. Flynn et ul.

[5 7] ABSTRACT To provide for better energy definition in a stigmatically focussing mass spectrometer with directional and energy focussing, the ions to be analysed travel consecutively through an entry diaphragm, a first analyser, a diaphragm to limit the energy range (energy diaphragm), a second analyser, (the energy dispersion factor of which is equal but opposite to that of the first analyser) and an exit diaphragm. One analyser is a momentum analyser operating with a wedge-shaped magnetic field, and the other analyser is an energy analyser operating with an electric field, The momentum analyser comprises a magnet having mutually opposite plane pole-piece surfaces which are mutually inclined and have straight edges which are located at the interval A parallel to the intersection line of the planes containing the pole-piece surfaces. A device is provided for adjusting the magnetic field intensity B at the interval A 2.10A from the intersection line to the value B L35 p/Ae (e charge of the ions; p momentum of the ions to be detected). The diaphragms between which the impulse analyser is placed are arranged on the intersection line with a mutual interval of approximately 1.28 A, and the energy analyser, which forms a stigmatic image of the entry side one of the two diaphragms between which it is arranged upon the exit side one of the two said diaphragms, contains a spherical condenser or a cylindrical condenser through which the ions travel in planes which pass through the cylinder axis.

4 Claims, 7 Drawing Figures STIGMATICALLY IMAGING DOUBLE FOCUSING MASS SPECTROMETER The present invention relates to a stigmatically imaging mass spectrometer with directional and energy focussing, wherein the ions to be analysed travel consecutively through an entry diaphragm, a first analyser, a diaphragm to limit the energy range (energy diaphragm), a second analyser, the energy dispersion factor of which is equal but opposite to that of the first analyser and an exit diaphragm one analyser is a momentum analyser operating with a wedge-shaped magnetic field, and the other analyser is an energy analyser operating with an electric field.

In a known double-focussing mass spectrometer of this type,-the momentum analyser consists of a magnetic lens which deflects the ions entering from an ion source through the entry slit through approximately 90. The ions of equal momentum leave the magnetic lens, acting as momentum analyser, in the form of a parallel beam, and after travelling through the energy diaphragm, by which there momentum range and hence the energy range is limited, enter a 45 spherical condenser which forms a stigmatic image of the ions of equal mass at a point in an image plane.

The above described known mass spectrometer has the disadvantage that the energy range (energy window) of the ions to be analysed cannot be adjusted satisfactorily because the ray path is telecentric in the region of the energy diaphragm (i.e., parallel ion beams travel through the energy diaphragm). Therefore any narrowing of the energy diaphragm involves an undesirable reduction in the transmission and hence in the sensitivity of detection of the mass spectrometer.

In my earlier application Ser.No. 32,931 filed 29 Apr. 1970 now US. Pat. No. 3,622,781 I have disclosed a mass spectrograph with double focussing, wherein between the entry slit and the energy diaphragm there is an electric image-forming arrangement by which an image of the entry slit is formed in the plane of the energy diaphragm. Although in the case of such a mass spectrograph the energy range can be adjusted by the energy diaphragm without appreciable impairment of the transmission, nevertheless the known spectrograph does not form a stigmatic mage; i.e., it focusses only in one plane and not at right angles thereto, so that the sensitivity here is impaired by the loss of ions which become lost due to the fact that no focussing occurs at right angles to the deflection plane.

It is an object of the present invention to provide a stigmatically imaging mass spectrometer wherein the energy range can be limited in the desired manner without impairing the sensitivity of detection.

SUBJECT MATTER OF THE PRESENT INVENTION A combination of known image-forming and lens systems is provided exhibiting dispersion, which systems are however specifically dimensioned and used in a novel mutual relationship.

According to a preferred embodiment of the invention, a stigmatically imaging mass spectrometer has a momentum analyser which contains a magnet with mutually opposite plane pole-piece surfaces which are mutually inclined and have straight edges which lie at the interval A, parallel to the intersection line of the planes containing the pole-piece surfaces, and also a device for adjusting the magnetic field intensity B, at the interval A 2.10A,, from the intersection line to the value B 1.35 p/Ae (e charge of the ions p momentum of the ions to be detected).

The diaphragms between which the momentum analyser is placed are arranged on the intersection line with a mutual interval of approximately 1.28A, and the energy analyser, which forms a stigmatic image of the entry-sided one of the two diaphragms between which it is arranged upon the exit-sided one of the two said diaphragms, contains a spherical condenser or a cylindrical condenser through which the ions travel in planes which pass through the cylinder axis.

In case a spherical condenser is used as energy analyser, the spherical condenser is preferably combined with a circular electric lens, preferably an Einzel-lens.

The energy analyser is preferably arranged between the entry diaphragm and the energy diaphragm, and the momentum analyser is placed between the energy diaphragm and the exit diaphragm.

The present stigmatically imaging mass spectrometer is particularly suitable for the analysis of ions with relatively strongly scattered initial energies, such as occur, e.g., when material is sputtered by means of' an ion probe.

Exemplary embodiments of the invention will be explained in further detail hereinbelow with reference to the drawing, wherein:

FIG. 1 shows a schematic plan of a first exemplary embodiment of a mass spectrometer according to the invention;

FIG. 2 shows a side elevation, partly sectioned, in a plane 22 of FIG. 1;

FIG. 3 shows schematic plan of a mass spectrometer according to a second exemplary embodiment of the invention;

FIG. 4 shows a partly sectioned side elevation along a second 44 in FIG. 3;

FIG. 5 shows a schematic plan of a mass spectrometer according to a third exemplary embodiment of the invention;

FIG. 6 shows a side elevation in the direction of the arrows 66 in FIG. 5, and

FIG. 7 shows a sectional elevation in a plan 77 of F1. 5.

The exemplary embodiment of the invention illustrated in FIGS. 1 and 2 contains a momentum analyser which operates with a magnetic field which is generated between two plane mutually inclined pole-piece surfaces which form a wedge shaped magnetic field gap. As the path arrows indicate, the field lines are circular arcs, the centres of which lie on the intersection line 10 of the planes containing the two pole-piece surfaces II. The field intensity between the pole-pieces is inversely proportional to the interval from the intersection line 10. In such a field, charged particles describe cycloidal paths.

It is known (I-I. Liebl, J. Appl. Phys. 38 (1967) 5277 to 5283), that such a field focusses a beam of charged particles having a momentum p, which start from a point on the intersection line and enter the magnetic field at right angles in the mean, after deflection through back onto a point on the intersection line if the field intensity is adjusted so that at the interval A 2.1OA from the intersection line it has the value 8,, 1.35p/Ae In the above formula: e charge of the particles, p momentum of the particles A interval of the intersection line from the straight edges of the pole-piece surfaces parallel to itself.

The focussing occurs in second approximation, and the interval of the focussing point from the source point is 1.28 A.

The momentum dispersion y(Ap), which is the interval of the focussing points of ions having the momentum difference Ap, is in this case y(Ap) 2.06/1 Ap/p Therefore in the exemplary embodiment of the invention illustrated in FIGS. 1 and 2, ions of momentum p which are made to form an image at a point 13 on the intersection line by the energy analyser which is to be described hereinbelow, are focussed by the magnetic field at a point 12 which lies in the exit diaphragm 14, likewise arranged on the intersection line 10, behind which in practice an ion detector device not shown e.g., a secondary electron multiplieris placed.

As energy analyser in the exemplary embodiment according to FIG. 1 a spherical condenser is used, which is likewise known in principle (see e.g., German Pat. No. 651,008; Z.Naturf. 12a (1957) 28-33).

The energy dispersion y(AU) of a spherical condenser in the Y direction i.e. the interval, calculated along the Y coordinate (see FIG. 1), between the focussing points of ions whose energies differ by AU, is

(3) wherein: U volt energy (corresponding to th acceleration voltage) of a particle which travels on the median spherical surface 20 in the field of the spherical condenser, AU deviation of the volt energy of a charged particle from U,,, U,, potential of the median spherical surface 20, L interval of the focussing point from the line 21 bisecting the angle of the spherical condenser in the direction of the exit axis 22, I sector angle of the spherical condenser, U and AU are referred to field-free space.

In order to obtain double focussing i.e., both directional and energy focussing the energy dispersion of the spherical condenser must be made equal but opposite to that of the magnetic field. The energy dispersion y,,,(AU) of the magnetic field for a specific mass is equal to half the momentum dispersion, therefore y,,.(AU) 1.03.4 AU/U The condition for double focussing is therefore L sin D/(l-U,,./U,,) 1.03/1

and in the case of symmetrical earthing of the spherical condenser (U,,,=O)

L sin l 1.03/1

In the mass spectrometer illustrated in FIGS. 1 and 2, the ions to be analysed, which may have been generated, e.g., by sputtering by means of an ion probe, enter through an entry diaphragm 24 and then pass through a diaphragm 26 limiting the aperture angle and the electric field of the spherical condenser 15, by which ions of equal energy in the ion beam entering through the entry diaphragm 24 become focussed upon a point (e.g., the point 13) on the intersection line 10. The energy diaphragm 28 is placed in the focussing plane in which the focussing points for the various energies lie. The ions then enter the magnetic field between the pole-piece surfaces 11, and ions whose momentum satisfies the equation (1) become focussed upon a point 12 in the aperture of the exit diaphragm 14. The type of ions to be detected can be selected by adjusting the magnetic field intensity B For a specific field intensity, only such ions can pass through the exit diaphragm 14 at any time as satisfy the condition of equation (I) i.e., ions which exhibit a specific ratio of mass to charge.

FIGS. 3 and 4 schematically illustrate an exemplary embodiment in which the energy analyser contains a 45 spherical condenser 15' which is preceded by a circular ion lens 30. The refractive power of this ion lens is adjusted by means of the potential of the centre electrode 32 so that an image of the source point in the entry diaphragm 24 is formed in the plane of the energy diaphragm 28. In other respects the construction of this embodiment corresponds to the exemplary embodiment according to FIGS. 1 and 2, and corresponding parts are designated by the same reference numerals.

In the case of symmetrical grounding of the spherical condenser 15, since sin \/2/2, we have L 1.46/1.

Further particulars of this combination of a 45 spherical condenser with a circular ion lens are described in J. Appl. Phys. 38 (1967) 5277-5283.

In the exemplary embodiments according to FIGS. 5 to 7, the energy analyser consists of a cylindrical condenser through which the ions travel in planes which pass through the cylindrical axis (cylindrical mirror analyser). Such analysers are known in principle, see., e.g., Z.Phys. 147 (1.957) 228 to 240 and Rev. Sci. Instr. 38 (1967) 1210 to 1216.

The energy dispersion AZ of the cylindrical mirror condenser 15" (FIGS. 5 and 6) in the direction of the cylinder axis 34 is AZ a cos 0/sin 0 (2K cos 0 l) AU/U,

(8) where 0 ta/ h) g (M wherein U and AU have the above connotations, 6 angle between the cylinder axis 34 and the axis 36 of the entering ion beam in field-free space, a radius of the inner cylinder 38, b radius of the outer cylinder 40, U,, potential of the outer cylinder 40.

The energy dispersion in the Y direction (direction at right angles to the axis of the exiting beam) is v(AU) AZ sinO a cos6/sin 9(2K,,cos l) AU/U The condition for double focussing in this case is therefore a cos6/sin 6(2K cos 01) 1.03/1

FIG. illustrates a particularly favourable special case. For by choosing K 1.31 and 0 42.3", directional focussing occurs in second approximation. The interval Z between the source point 42 in the entry diaphragm 24 and the image point 13 on the intersection line is then 6.120. The condition for double focussing in this case is FIG. 5 therefore represents a mass spectrometer consisting of mechanically simple components which is double focussing in first approximation and forms a stigmatic image in second approximation.

In other respects this embodiment also corresponds to that according to FIGS. 1 and 2.

I claim:

1. Stigmatically imaging mass spectrometer with directional and energy focussing, comprising consecutively positioned with respect to ion travel:

an entry diaphragm (24);

a first analyzer;

an energy diaphragm (28) to limit the energy range;

a second analyzer having an energy dispersion factor which is equal to and opposite to that of the first analyzer; and an exit diaphragm (14); one of the analyzers being a momentum analyzer having a wedge-shaped magnetic field, and the other analyzer being an energy analyzer having an electric field; I

the momentum analyzer comprising a magnet having mutually opposite plane pole-piece surfaces (11) which are mutually inclined and have straight edges which are parallel and spaced by a distance A, from the intersection line (10) of the planes containing the pole-piece surfaces (11) and parallel to said intersection line, and the magnetic field intensity 8,, at the distance interval being defined y from the intersection line ('10) being adjusted to the value 8,, 1.35p/Ae, wherein e charge of the ions; p momentum of the ions to be detected; the diaphragms (14, 28) between which the momentum analyzer is placed are arranged on the intersection line (10) with a mutual spacing interval of approximately 1.28 A;

the energy analyzer, which forms a stigmatic image of the diaphragm (24) at the entry side thereof on the diaphragm (28) located at the exit side thereof, including a spherical condenser (l5, 15) or a cylindrical condenser (15"), the ions travelling through the condenser in planes which pass through the cylinder axis (34), ions of equal energy being focussed on the energy diaphragm (28).

2. Mass spectrometer according to claim 1, wherein the spherical condenser (15) is a symmetrically grounded condenser (15) having an entry axis, said entry axis being oriented at the distance L 1.03A to be parallel to the intersection line (10) of the planes passing through the pole-piece surfaces (11).

3. Mass spectrometer according to claim 1 wherein the energy analyzer includes a 45 spherical condenser (15') and a circular ion lens (30) located in advance of said spherical condenser.

4. Mass spectrometer according to claim 1, wherein the energy analyzer includes a cylindrical condenser (15") having an axis, an outer cylinder (40) and an inner cylinder (38), the axis (34) subtending an angle 0 of 42.3 with the axis (36) of the entering ion beam;

wherein the radius -aof the inner cylinder (38) is equal to 0274A, and wherein the energy of the entering ions and the voltage of the outer cylinder, and its radius have the relationship:

(U /U log (b/a) 1.31, wherein U, volt energy of an entering ion U =voltage of the outer cylinder (40) -aradius of the inner cylinder (38) b radius of the outer cylinder (40). 

1. Stigmatically imaging mass spectrometer with directional and energy focussing, comprising consecutively positioned with respect to ion travel: an entry diaphragm (24); a first analyzer; an energy diaphragm (28) to limit the energy range; a second analyzer having an energy dispersion factor which is equal to and opposite to that of the first analyzer; and an exit diaphragm (14); one of the analyzers being a momentum analyzer having a wedgeshaped magnetic field, and the other analyzer being an energy analyzer having an electric field; the momentum analyzer comprising a magnet having mutually opposite plane pole-piece surfaces (11) which are mutually inclined and have straight edges which are parallel and spaced by a distance Ao from the intersection line (10) of the planes containing the pole-piece surfaces (11) and parallel to said intersection line, and the magnetic field intensity BA at the distance interval being defined by A 2.10Ao from the intersection line (10) being adjusted to the value BA 1.35p/Ae, wherein e charge of the ions; p momentum of the ions to be detected; the diaphragms (14, 28) between which the momentum analyzer is placed are arranged on the intersection line (10) with a mutual spacing interval of approximately 1.28 A; the energy analyzer, which forms a stigmatic image of the diaphragm (24) at the entry side thereof on the diaphragm (28) located at the exit side thereof, including a spherical condenser (15, 15'') or a cylindrical condenser (15''''), the ions travelling through the condenser in planes which pass through the cylinder axis (34), ions of equal energy being focussed on the energy diaphragm (28).
 2. Mass spectrometer according to claim 1, wherein the spherical condenser (15) is a symmetrically grounded 90* condenser (15) having an entry axis, said entry axis being oriented at the distance L 1.03A to be parallel to the intersection line (10) of the planes passing through the pole-piece surfaces (11).
 3. Mass spectrometer according to claim 1 wherein the energy analyzer includes a 45* spherical condenser (15'') and a circular ion lens (30) located in advance of said spherical condenser.
 4. Mass spectrometer according to claim 1, wherein the energy analyzer includes a cylindrical condenser (15'''') having an axis, an outer cylinder (40) and an inner cylinder (38), the axis (34) subtending an angle theta of 42.3* with the axis (36) of the entering ion beam; wherein the radius -a- of the inner cylinder (38) is equal to 0.274A, and wherein the energy of the entering ions and the voltage of the outer cylinder, and its radius have the relationship: (Uo/Ub) log (b/a) 1.31, wherein Uo volt energy of an entering ion Ub voltage of the outer cylinder (40) -a- radius of the inner cylinder (38) b radius of the outer cylinder (40). 